Normal tissue homeostasis is maintained by adult stem and progenitor cells that use cell intrinsic mechanisms and information from their microenvironment to execute a very complex specialized type of mitosis known as asymmetric cell division. Two resulting from this division daughter cells acquire different cell fates. While one cell continues proliferation, another daughter cell withdraws from the stem cell niche, remodels its intercellular interactions and starts a program which ultimately leads to the cell cycle withdrawal and differentiation. This process ensures that only the necessary number of cells is produced at any given time and the cell death and depletion in each tissue is balanced by the birth of new cells. Defective asymmetric cell division can result in the failure of normal tissue homeostasis and birth of cancer stem cells, which cannot produce daughters that can properly withdraw from the cell cycle and differentiate. While asymmetric cell division plays a pivotal role in tissue homeostasis, little is known about its mechanisms in mammalian organisms. Studies in model organisms revealed critical role of intercellular interactions and apical-basal cell polarity in regulation of asymmetric cell division. In Drosophila, basal cell polarity gene lethal giant larvae (lgl) was identified as an important regulator of asymmetric cell division, and mutations in lgl result in overproduction of dividing stem cells and development of cancer. Mammalian genomes contain two lgl orthologs Llgl1 and Llgl2. While previous studies of LLGL1 and LLGL2 in cancer cell lines indicated their potential role as tumor suppressors, genetic analysis of these genes in mice was complicated by the genetic redundancy and lethality of Llgl mutants. We have now generated conditional, tissue specific Llgl1/2 double knockout mice. Our preliminary experiments revealed tumor-suppressive function of Llgl1/2 in skin squamous cell carcinoma. In this grant, we propose to use our mutant animals, primary cell cultures and in utero lentiviral transduction of skin epidermis to reveal the role and molecular mechanisms of Llgl gene family in stem and progenitor cells during normal tissue homeostasis and cancer. These studies will help to understand the mechanisms of mammalian stem and progenitor cells self-renewal and differentiation and the role of these mechanisms in cancer.